Internal combustion engines of two-stroke or four-stroke type usually are equipped with a fuel supply system of carburetor type or injection type. In a carburetor, the throttle of the carburetor is affected by the operator's demand, so that wide open throttle produces a minimum throttling in the carburetor barrel. The depression created by the passing air in the carburetor venturi draws fuel into the engine. Traditionally, carburetor engines are equipped with stationary nozzles or manually adjustable nozzles to regulate the degree of richness/leanness of the air-fuel mixture. As the demands on lower fuel consumption jointly with demands on cleaner exhaust have increased also electronically controlled nozzles have been suggested. In the latter case the amount of fuel supplied to the carburetor barrel is adjusted. This is affected with the aid of variable throttling. Increasing throttling gives a leaner air-fuel mixture. The throttling is regulated continuously or in small steps. However, such quantity adjustment is comparatively complicated and expensive. It is already known to provide for a brief shut-off during the suction phase in order to reduce the amount of fuel or, in accordance with the teachings of DE 23 48 63S, to briefly open a normally closed valve during the suction phase. It is very difficult to rapidly open and close a valve, or vice or vice versa, with accuracy. The carburetor is positioned in an intake passage leading to the engine cylinder. This intake passage is opened and closed by the engine piston or by a particular valve, usually called suction valve. Owing to this opening and closing of the intake passage varying flow speeds and pressures generate inside the passage. Since the carburetor is constructed to allow the depression in the carburetor barrel to draw in fuel, also the amount of fuel supplied will be largely affected by the closing and the opening of the intake passage. The basic function of the carburetor is to add an appropriate amount of fuel to a predetermined amount of passing air.
EP 0 799 377 a method characterized primarily in that in the fuel supply system shut-off is effected during a part of the operating cycle by means of a shut-off valve shutting off the entire fuel flow or a part flow, and in that the shut-off is arranged to take place to an essential extent during a part of the operating cycle when the intake passage is closed and consequently the feed of fuel is reduced or has ceased. This means that the amount of fuel supplied can be precision-adjusted by a slight displacement of one of the flanks of the shut-off valve shut-off curve.
However, precision-adjusting the fuel supply by a slight displacement of one of the flanks of the shut-off valve shut-off curve still requires a comparably high accuracy of the shut-off valve. Further a steeper slope of the flank provides for finer the fuel adjustments, i.e. the time for the shut-off valve to change from open to close or vice versa; but a quicker shut-off valve is more expensive.
EP 0 799 377 suggest the shut-offs to be done for each revolution varying the fuel supply by adjusting the displacement of the flank of the shut-off valve; but in particular for crank case scavenged two/four-stroke engines, the shut-offs can be performed every other, every third or possibly every forth engine revolution instead upon each engine revolution, in the case of a four-stroke engine, half as often. In that case a major fuel amount adjustment is made instead, for instance by completely shutting of the fuel supply for a revolution. This can be done since the crank case in crank case scavenged two-stroke engines or crank case scavenged four-stroke engines can hold a considerable amount of fuel and consequently serve as a leveling reservoir, it is therefore not necessary to adjust the fuel supply for each revolution when controlling the fuel supply to the engine, i.e. adjusting the fuel supply in one revolution will affect the subsequent revolutions.
By shutting off the entire fuel supply for a revolution, the requirements of accuracy and speed of the shut-off valve could be much reduced, however, utilizing the method of EP 0 799 377, a very rough regulation would be provided, i.e. for the two-stroke engine the sequences; 1/2, 1/3, 1/4 corresponds to the fuel reductions steps 50%, 33% and 25% and for the four-stroke engine the sequences; 1/2, 1/4, 1/6, 1/8 corresponds to the fuel reductions steps 50%, 25%, 17%, 13%. The difference in fuel reduction between fuel shut-offs every second and every third revolution is as high as 17 percentages units and between fuel shut-offs at every third and every fourth revolution, the difference is still as high as 8 percentages units. These differences could of course be compensated for by varying the displacement of one of the flanks of the shut-off valve shut-off curve, but then the requirements of the shut-off valve increases.
Further, each time the shut-off valve is activated energy is consumed, thus it would be advantageous providing a control method minimizing the number of opening and closings of the shut-off valve, without compromising with the accuracy of the control method.